USB: clarify USB_DT_USB_SSP_CAP_SIZE(ssac) definition

USB_DT_USB_SSP_CAP_SIZE(ssac) gives the size of the SSP capability
descriptor. The descriptor consists of 12 bytes plus a array of
SSA entries.

The number of SSA entries is stored in a SSAC value in the first 12 bytes,
The USB3.1 specification 9.6.2.5 defines SSAC as zero based:
"The number of Sublink Speed Attributes = SSAC + 1." This is not
intuitive and has already caused some confusion.

Make a small modifiaction to the USB_DT_USB_SSP_CAP_SIZE(ssac)
definition to make it a bit clearer

Signed-off-by: Mathias Nyman <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/uapi/linux/usb/ch9.h

···
 #define USB_DT_USB_PTM_ID_SIZE		3
 /*
  * The size of the descriptor for the Sublink Speed Attribute Count
- * (SSAC) specified in bmAttributes[4:0].
+ * (SSAC) specified in bmAttributes[4:0]. SSAC is zero-based
  */
-#define USB_DT_USB_SSP_CAP_SIZE(ssac)	(16 + ssac * 4)
+#define USB_DT_USB_SSP_CAP_SIZE(ssac)	(12 + (ssac + 1) * 4)
 
 /*-------------------------------------------------------------------------*/